Method for converting a floatable barge into a semi-submersible vessel

ABSTRACT

Methods and apparatus are provided for converting a floatable barge into a semi-submersible vessel. An independently floatable cradle unit is provided which has at least two submersible hulls, a plurality of support columns, transverse braces, and a buoyancy mechanism for selectively ballasting and de-ballasting the submersible hulls. The cradle unit is sunk or at least partially submerged and a barge is floated thereabove, whereupon the cradle is raised to lift the barge from the water. The barge is rigidly connected to the upper portions of the support columns. An inclined lay ramp and/or derrick may be mounted on the cradle to convert the barge into a semi-submersible lay or derrick vessel.

United States Patent [1 1 McClure 1 1 METHOD FOR CONVERTING A FLOATABLEBARGE INTO A SEMI-SUBMERSIBLE VESSEL [75] Inventor: Alan C. McClure,Houston, Tex.

[73] Assignee: Brown & Root, Incorporated,

Houston, Tex.

[22] Filed: Apr. 6, 1973 [21] Appl. No; 348,594

Lloyd 1 14/115 D X 1 1 July 15, 1975 3,797,438 3/1974 Fayren .1 114/05 DPrimary Examiner-Trygve M. Blix Ari-(slum E.raminerCharles E. FrankfortAltorney, Agent. or Firm-Burns. Doane, Swecker & Mathis 5 7] ABSTRACTMethods and apparatus are provided for converting a floatable barge intoa semi-submersible vessel. An independently floatable cradle unit isprovided which has at least two submersible hulls, a plurality ofsupport columns, transverse braces, and a buoyancy mechanism forselectively ballasting'and de-ballasting the submersible hulls. Thecradle unit is sunk or at least partially submerged and a barge isfloated there above, whereupon the cradle is raised to lift the bargefrom the water. The barge is rigidly connected to the upper portions ofthe support columns. An inclined lay ramp and/or derrick may be mountedon the era die to convert the barge into a semi-submersible lay orderrick vessel.

7 Claims, 13 Drawing Figures ymgmqmm 1 5 ms 3 a 94'. 503

SHEET AW .ii' i METHOD FOR CONVERTING A FLOATABLE BARGE INTO ASEMI-SUBMERSIBLE VESSEL BACKGROUND AND OBJECTS OF THE INVENTION Thisinvention relates to semi-submersible vessels, and more particularlyconcerns the conversion of a conventional floatable barge into asemi-submersible vessel.

Various offshore naval operations are being performed in increasingfrequency in rough water areas throughtout the world. In the North Seaand Australia, for example, extensive oil drilling and pipeline layingoperations are being conducted. The severe wave conditions typicallyencountered in areas like these greatly limit effective vesselutilization.

Vessels having hull configurations which are characterized by largewater plane areas, such as conventional floating barges, for example,are subjected to excessive motion in heave, pitch, and roll directionsduring rough sea conditions. Consequently, vessels such as these mightencounter substantial difficulties during the performance of operationssuch as pipeline laying, ship to ship material transfer, hoistingoperations, platform erection, and other operations which require steadyworking decks. The dangers produced by such unstable conditions canthreaten the safety of the crew as well as result in costly delay.

Vessels of the semi-submersible type, on the other hand, typicallyexhibit a high degree of stability and thus are being widely employed inrough waters. Semisubmersible vessels typically utilize a pair ofbuoyant hulls connected below a working platform by means of verticallydisposed support columns. The buoyancy of the hulls may be varied toenable the working platform to be selectively raised and lowered inrelation to the water surface. In this manner, the buoyant hulls can besubmerged to a position below the water surface while maintaining theworking platform above the water surface at a higher level than themaximum expected wave heights.

Thus, contact of the semi-submersible with the water surface, where waveaction is most severe, is of a limited nature, occuring solely at thevertically projecting support columns. Consequently, thesemi-submersible offers minimal obstruction to wave action such that thenatural periods of motion of the semi-submersible, during heave, pitch,and roll, are longer than similar periods of normally encountered waves.The resultant stability of the working platform facilitates theperformance of sea-based operations, in spite of the presence of a roughwater environment.

Numerous problems are involved in satisfying the growing need forsemi-submersible vessels to carry out the tasks required in rough-wateroperations. Among these problems are the considerable time and expensesencountered in constructing a sufficient number of semi-submersibles. Itwould, therefore, be highly advantageous to provide semi-submersiblevessels quickly and economically.

One proposed form of semi-submersible is illustrated in US. Pat. No.3,575,005, issued on Apr. 13, 1971. This patent proposes a method oftransforming a conventional ship-type vessel into a semi-submersible. Itwould be desirable, however, that a conventional vessel be transformedinto a semi-submersible in a manner which requires less vesselmodification while providing maximum vessel stability and utilization.

It is, therefore, a general object of the invention to provide novelmethods and apparatus for obviating or substantially alleviatingproblems of the types previously discussed.

It is a particular object of the invention to devise novel methods andapparatus for providing semisubmersible vessels at considerable savingsof time and cost.

It is another object of the invention to provide novel methods andapparatus for modifying the hull characteristics of conventionalfloatable barges to minimize the effects of wave action thereon.

It is still a further object of the invention to provide novel methodsand apparatus for converting conventional floatable barges intosemi-submersible vessels in a manner requiring minimal vesselmodification while providing optimum vessel stability.

It is still another object of the invention to provide such novel bargeconversion methods and apparatus wherein substantial portions of theconversion operation may be accomplished independently of a physicalpresence of the barge.

It is yet another object of the invention to provide novel methods andapparatus for converting conventional floatable barges intosemi-submersible lay vessels in a manner requiring minimal vesselmodification while providing optimum vessel stability.

BRIEF SUMMARY OF PREFERRED EMBODIMENTS OF THE INVENTION At least some ofthe foregoing objects are intended to be accomplished by the provisionof barge conversion methods and apparatus in accordance with thisinvention.

In one preferred method aspect of the invention a floatable barge isconverted into a semi-submersible vessel. In effecting this conversion,a cradle unit is provided which has a pair, or more, of preferablyparallel submersible hull members, a plurality of columns extendingtransversely upwardly from each hull, transverse bracing structurespreferably interconnecting columns of one hull with opposing columns ofthe other hull, and a buoyancy mechanism for selectively ballasting andde-ballasting the hull members.

The buoyancy mechanism is actuated to cause the cradle unit to sink, atleast partially in a body of water, such that the hulls are submerged,the bracing structures define support surface means which may behorizontal and be disposed below the water surface, and the opposingcolumns define a barge-receiving zone therebetween.

With the cradle thus disposed, the barge is floated on the body of waterto a position adjacent the bargereceiving zone, with the underside ofthe barge located above the horizontal support surface means. Thebuoyancy mechanism is then actuated to bring the horizontal supportsurface means into supporting contact with the underside of the bargeand lift the barge from the surface of the body of water.

Rigid connection is made between the barge and the upper portions of thecolumns.

The bracing structures include a plurality of brace beamsinterconnecting opposing columns, and diagonal brace elements.

The buoyancy mechanism includes ballast pumping equipment in each hullwhich communicate by a conduit extending through a brace beam.

The barge and cradle are interconnected by extension frames. Theseframes may include upper surfaces which define extensions of the bargehull and working platforn. and may serve to support at least part of anoffset rotary crane.

During the conversion operation, the barge may be floated to a positionadjacent the barge-receiving zone, yet off-set from the center thereof.An extension frame may be provided to serve as an auxiliary workingdeck.

In an independently significant aspect of the invention the cradle mayinclude an inclined or downwardly curved lay ramp which is suitable forthe laying of an elongate flexible structure, such as pipeline, from thevessel. The lay ramp may be supported by columns at one side of thecradle, the columns having rampsupporting portions of varying height inaccordance with the ramp inclination.

The after-most column adjacent the ramp may be provided with a hitchmechanism for supporting a conventional stinger mechanism Furtheraspects of the invention relate to combinations of structural elementsfor implementing the previously described methods.

DRAWINGS Other objects and advantages of the present invention may beunderstood from the following detailed description of preferred formsthereof with reference to the accompanying drawings in which likenumerals refer to like elements and in which;

FIG. 1 is a schematic sectional view of a barge conversion assemblytaken along the line 2-2 in FIG. 2 with the tower portion removed,according to one preferred embodiment of the subject invention;

FIG. 2 is a schematic side elevational view of the barge conversionassembly including the tower portion;

FIG. 3 is a schematic front elevational view of the barge assemblydepicted in FIG. I;

FIG. 4 is a schematic front elevational view of another preferred bargeconversion assembly according to the invention, with the tower portionremoved;

FIG. 5 is a schematic front elevational view of a further preferredembodiment according to the invention, with derrick and tower portionsremoved;

FIG. 6 is a schematic sectional view of the barge conversion assemblytaken along line 66 in FIG. 7;

FIG. 7 is a schematic side elevational view of the barge conversionassembly depicted in FIGS. 4 and 6, including the derrick and towerportions;

FIG. 8 is a schematic longitudinal cross-sectional view through asubmersible buoyant hull member forming part of the barge conversionassembly of the invention;

FIG. 9 is a transverse sectional view through a support column, takenalong the line 9-9 in FIG. 2', and

FIGS. 10 through 13 are schematic front elevantional views sequentiallydepicting various steps in the barge conversion operation of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with thepresent invention a barge conversion assembly is provided, comprising aconventional floatable barge and a buoyant cradle unit attachablethereto to form a semi-submersible vessel.

One form of the barge conversion assembly is illustrated schematicallyin FIGS. 1, 2, and 3 and comprises a barge l0 and a buoyant cradle unit12. The barge 10 is a conventional floating derrick barge having front,rear, and side portions 16, I8, and 20 and a working deck or platform 22carried thereby.

Operably disposed on the working deck 22 is a conventional derrick orcrane unit 24. The crane includes a boom 25 which is rotatably mountedon a tub 26 that is secured to the deck 22. Only a tub portion of thecrane is depicted in FIG. 1 to indicate the relative location thereof.In this connection, the rotational axis of the crane is arranged so asto intersect the centerline of the barge. Located at the front of thebarge is a control tower 28 from which various barge activities aredirected.

It will be appreciated that the barge, when in a floating condition,exhibits a relatively large water plane area, thereby rendering thebarge highly sensitive to wave action. Thus, in rough water areas, orwhere long swells are prevalent, motions of the barge in heave, pitch,and roll directions will tend to restrict the usefulness of the barge.

The cradle unit 12 includes twin submersible hulls, preferably in theform of buoyant caissons or pontoons 3UP and 308 which are oriented onport and starboard sides in parallel relationship. The pontoons includeballast chambers and equipment for selectively ballasting andde-ballasting these chambers with sea water, as will be subsequentlydescribed.

More specifically, each of the pontoons carries a plurality of supportcolumns 32 and 34 which extend transversely relative to the longitudinalaxes of the pontoons. While the preferred embodiment described inconnection with FIGS. 1, 2, and 3 discloses three columns associatedwith each pontoon, it will be appreciated that any suitable number ofcolumns may be employed.

Between corresponding, or opposite, ones of the fore, aft, andintermediate (amidships) support columns 32F, 32A, 32!, and 34F, 34A,34] are provided horizontal transverse bracing assemblies 35F, 35A and35I. Each bracing assembly includes a lower horizontal brace beam 36 andan upper horizontal brace beam 38 secured between oppositely disposedones of the columns 32 and 34. A plurality of diagonal brace members40,41,42 and 43 preferably four in number, are secured at their upperends to the upper brace beam 38. The two inner diagonal braces 41 and 42are connected at theirlower ends to the lower brace beam 36. The twoouter diagonal braces 40 and 43 are connected at their lower ends to theopposed support columns 32 and 34, respectively.

With the pontoons 30, the columns 32 and 34, and the bracing assemblies35 in an assembled condition, the cradle constitutes a unitary floatablestructure.

Although the manner of assembling the barge and cradle unit togetherwill be described subsequently in greater detail, it is noted that thecolumns 32, 34 are secured to the barge by means of steel frameextension structures 46 which are rigidly secured between the barge andthe columns. In their assembled condition, the frame structures 46define extensions of the barge hull.

Referring particularly to FIG. 8, it will be understood that thepontoons 30S and 30A are substantially identical. Each pontoon isprovided with a pump room 48. Within the pump room are disposed aplurality of pumps 52 and 54 which are suitably connected, viaappropriate piping and valve manifolds, between a sea chest 56 and anumber of water-tight ballast chambers 58. The pumps 52 and 54 areoperable to regulate the transfer of sea water to the ballast chambersto alter the buoyancy of the pontoons. Additionally, the pump 54 isarranged to transfer ballast water from one pontoon to the other througha conduit 60 extending through the lower horizontal brace beam 36 of theintermediate bracing assembly. in this fashion, the pump mechanism ofone pontoon may, if necessary, be utilized to control the buoyancy ofboth pontoons.

The control room of the control tower 28 is provided with a main ballastcontrol console (not shown) for remotely operating the ballast pumps andtheir associated valves. Auxiliary ballast controls are desirablydisposed in the pontoon pump rooms and are capable of being operatedindependently of the main control console if such necessity arises.

The pontoons also include chambers 59 and 61 for the storage of freshwater and fuel, respectively. Suitable pumps may be provided forconducting this water and fuel to the barge. Such an arrangement tendsto maximize usage of the barge storage facilities.

As can be observed in FIG. 9, the intermediate ones 321 and 341 of thesupport columns include central tubes 62. These tubes provide shearstrength across the columns and accommodate access to the pontoon pumprooms via appropriately arranged ladders. The fore and aft supportcolumns 32F, 32A, and 34F, 34A are also provided with central tubes tofacilitate passage of piping between the barge and the pontoons.

The tubes are held in fixed relation within the col umns by means offore-aft bulkheads 64 and transverse bulkheads 66.

CONVERSION OPERATION Conversion of a floating barge into asemisubmersible vessel in accordance with the invention is initiated byfabricating the cradle unit 12. In this connection, and as can be viewedin FIGS. through 13, the pontoons S and 30F are constructed and areprovided with stub portions 70 of the support columns 32 and 34.

The pontoons are then launched into a floating condition. While afloat,the pontoons are interconnected by installation of the lower horizontalbrace members 36. The remaining portions of the support coluns 32 arethen erected, defining a barge-receiving zone 72 therebetween.Installation is completed of the remainder of the transverse bracingassemblies 35F, 35A, and 35l, including the upper transverse brace beams38 and the diagonal braces 40, 41, 42 and 43.

Portions of the hull. extension frames 46 are then welded onto the upperends of the support columns.

As the above-described construction of the extension frames is beingcarried out, the piping systems, machincry, and wiring are installedwithin the pontoons, support columns, and hull extension frames.

The cradle unit is then transported to a deep water location where thepontoons 30S and 30P are ballasted causing the cradle unit to sink to adepth sufficient to permit the barge 10 to be floated into thebargereceiving zone 72. With the barge 10 in position, the pontoons willbe de-ballasted to raise the cradle unit and thus the brace beams 38into engagement with the underside of the barge (FIG. 12). The uppersurfaces of the brace beams 38 thus define a generally horizontal bargesupport surface means. The above-water portions of the extension frames46 are then connected.

Finally, the pontoons are further de-ballasted to raise the barge out ofthe water to a position where installation of the hull extension frames46 may be completed.

As an alternate procedure, portions of the extension frames 46 may beinitially constructed on the barge and then extended outwardly to thesupport columns.

To complete the conversion, the piping and wiring systems between thebarge and the cradle unit are interconnected.

Operation of the vessel as a semi-submersible will be substantially thesame as that of conventional semisubmersible vessels. For a detailedsummary of the operation of vessels of this type, reference may be hadto a paper entitled Development of the Project Mohole Drilling Platformdrafted and presented by the present inventor at an annual meeting ofthe Society of Naval Architects and Marine Engineers on Nov. 11-12,1965.

During semi-submersible operation of the vessel, the cradle unitprovides optimum support for the barge by means of the cross bracingassemblies 35. The bracing assemblies themselves, however, offer minimalobstruction to wave action and thus do not impair vessel stability andmovement.

LAY BARGE CONVERSION In another preferred embodiment of the invention aconventional floatable barge is converted into a semisubmersible layvessel suitable for the offshore laying of an elongate flexible element,such as pipeline (FIGS. 4, 6, and 7).

in accomplishing this conversion a cradle unit 79 is constructed. Thiscradle unit 79 includes a pair of port and starboard pontoons 8UP and808, a plurality of upstanding support columns 82F, 82A and 821, and84F, 84A, and 841 attached to the pontoons. Bracing assemblies 86,similar to 35 extend between corresponding, opposite support columns.Extension frames 88, 89, and 90 extend between the support columns andthe sides of the barge 78.

Significantly, the support columns 84 and the extension frame 90 arearranged to accommodate a lay ramp 92. The lay ramp 92 is downwardlyinclined in a foretoaft direction and is carried at the upper ends ofthe starboard support columns 34. The top outer edges of the supportcolumns 84A and 84l are disposed at levels of progressively diminishingheight in a fore-to-aft direction in accordance with the inclination ofthe ramp 92. That is, the support columns 84A and 84l are recessed attheir top outer edges to define shoulders 94 which support the lay ramp92. The forward, upper end of the lay ramp 92 is substantially flushwith the extension frame 90 and the deck of the barge. The lower end ofthe ramp terminates above the starboard pontoon S adjacent the expectedwater surface level.

The ramp may be of any suitable width, and may extend outwardly of thesupport columns 34. Accordingly, appropriately sized bracing flanges 98may be utilized to support the outwardly extending portion of the ramp.

The hull extension frame 90 extends along substantially the entirelength of the barge to define an auxiliary deck surface 95, which isflush with the working deck of the barge 78. This auxiliary deck can beused to carry equipment associated with pipeline laying, such as pipestorage units and welding equipment and other apparatus such asdescribed in the previously cited patents.

Known pipelaying techniques and equipment may be utilized in conjunctionwith a converted lay barge according to this invention. Exemplarythereof are the techniques and equipment described in Lawrence US. Pat.Nos. 3,390,532 (issued July 3, 1968), 3,472,034 (issued Oct. 14, 1969),3,487,648 (issued Jan. 6, 1970) and 3,585,806 (issued June 22, 1971),Rochelle U.S. Pat. No. 3,507,126 (issued June 13, 1972), and Jones etal. US. Pat. No. 3,668,878 (issued Apr. 9, 1969). The afore-mentionedpatents are assigned to the assignee of this invention and thedisclosures of these patents are incorporated herein by reference.

Moreover, additional below-deck quarters may be built into the hullextension frames to accommodate additional personnel, supplies andequipment.

Preferably, the aft starboard support column 34A is provided with ahitch structure 100 suitable for carrying a conventional stingermechanism (not shown) dur ing laying operations. Such a stinger andhitch are shown in Lochridge US. Pat. No. 3,606,759, issued on Sept. 21,1971 and assigned to the assignee of this invention. The disclosure ofthis Lochridge patent is incorporated herein by reference.

The procedure for converting the barge into a semisubmersible lay bargeis similar to that described in accordance with FIGS. 10 through 13. Inthis instance the pontoons 80S and 801 are constructed and floated. Theremaining portions of the cradle 79, including the lay ramp 92, are theninstalled. Subsequently, the conventional barge 78 is floated intoposition between the support columns, and is raised by the cradle unitsuch that the frame extensions 88, 89, and 90 may be connected betweenthe barge and the support columns. In this fashion, the barge isrendered capable of semisubmersible operation for the off-shore layingof elongated flexible elements, such as pipeline.

If desired, conventional auxiliary pipe-laying equipment including astinger unit and pipe-tensioner unit may be mounted on the cradle priorto the initiation of a pipe-laying operation.

It may be desirable to re-locate the crane 102 from its center-lineposition to a location P off-set toward the port side in order tocompensate for the added starboard weight imposed by the lay ramp andlay equipment. In order to provide maximum compensation, the crane maybe partially located on the frame extension 89 which is of a lengthsufficient to accommodate the crane. Such an arrangement also serves toincrease the effective reach of the crane over the port side.

An offset arrangement of the crane may also be utilized in connectionwith the barge conversion assembly described in connection with FIGS. 1,2, and 4, should it be desired to increase the outward reach of thecrane. Compensation for the offset forces imposed by the crane duringlifting operations may be provided by suitable ballasting of thesubmersible hulls 30S and 30F.

Although the preceding embodiments of the invention have been discussedin connection with a barge which is positioned substantially centrallybetween the support columns, it will be appreciated that the otherarrangements are possible in accordance with the teachings of thisinvention. One alternate arrangement is shown in FIG. 5 in which a bargeis seated upon shoulders 112 formed in the starboard support columns114. The barge is also supported by a plurality of brace assemblies 116and is connected to a frame extension 118 projecting from the portsupport columns 120.

The frame extension 118 defines an auxiliary working deck on whichvarious equipment, such as a crane, can be mounted. Also, additionalcrew quarters and storage compartments may be installed in the belowdeckportion of the frame extension 118. Assemblage of the structure shown inFIG. 5 is accomplished in a manner similar to the procedure discussed inconnection with FIGS. 10 through 13. In this instance, however, thebarge will be floated to an offset position adjacent the barge receivingzone, e.g., wherein its starboard side is situated over the shoulders112. Subsequent to being lifted by the cradle unit, the barge isconnected to the starboard support columns 1 14 and to the frameextension 118.

SUMMARY OF MAJOR ADVANTAGES AND SCOPE OF THE INVENTION As is apparent,the present invention provides novel methods and apparatus for enablinga conventional floatable barge to be effectively converted into asemisubmersible vessel. In this fashion, a pre-existing barge whichmeets structural and design standards required of sea-going vessels maybe transformed into a semisubmersible with only minimal amounts of bargemodification.

Significantly, most of the conversion efforts, i.e., construction of thecradle unit, may be accomplished independently of a physical presence ofthe barge. Thus, work may be advanced while the barge is engaged inother marine operations. Because of this and the rapid manner in whichthe final conversion operation is accomplished, use of the barge is lostfor only a minimal period.

The cradle unit is constructed in a manner which produces optimumstrength and stability, while presenting minimal obstruction to freetravel through the water. This is the result of the compact, yeteffective, bracing assemblies which rigidify the cradle elements into aunitary, independently floatable structure capable of withstandinghighly severe wave environments. The relatively skeletal arrangement ofthe bracing assemblies, however. assures that the stability andmaneuverability of the vessel will not be unduly impaired by waveaction.

The bracing assembly also provides a guide for enabling the ballastingsystems of the submersible hulls to be conveniently interconnected.

The present invention is also highly advantageous in affording rapidconversion of a conventional barge into a semisubmersible lay barge. Bymeans of a unique design, a lay ramp is carried by the cradle unit andis arranged to be operably associated with the barge working deck.

The provision of frame extensions which are rigidly connected betweenthe barge and the support columns enables the effective working deckarea of the barge to be extended, as well as enabling extra below-deckquarters to be added. This added area may be utilized to mount an offsetcrane, a feature which is particularly advantageous in counterbalancingthe barge.

Although the invention has been described in connection with a preferredembodiment thereof, it will be appreciated by those skilled in the artthat additions. modifications, substitutions and deletions notspecifically described may be made without departing from the spirit andscope of the invention as defined in the appended claims.

What is claimed is:

l. A method of converting a floatable barge having a working deck into asemi-submersible vessel comprising the steps of:

providing an independently floatable cradle unit having spacedhorizontally elongate submersible hull members defining port andstarboard submersible hull members,

a plurality of columns extending vertically upwardly from each of saidhull members to define port and starboard columns,

transverse bracing means interconnecting said port columns with saidstarboard columns, said bracing means comprising lower transversebracing, upper transverse bracing, and intermediate diagonal bracing,and

buoyancy means for selectively ballasting and deballasting saidsubmersible hull members;

actuating said buoyancy means to cause said cradle unit to partiallysink within a body of water while maintaining said cradle unitpositively buoyant, such that said submersible hull members arecompletely submerged,

said upper transverse bracing defines a horizontally disposed supportsurface means disposed below the surface ofthe body of water between theport and starboard submersible hull members, and

the upper ends of at least some of the columns of said submersible hullmembers extend above the water surface to define a barge receiving zonetherebetween;

floating said barge on the body of water into said barge receiving zonebetween said port and starboard columns to orient the underside of saidbarge over said support surface means, while maintaining said cradleunit positively buoyant;

actuating said buoyancy means for de-ballasting said hull members andincreasing the postive buoyancy thereof to:

raise said cradle unit in a manner bringing said support surface meansinto supporting contact with the underside of said barge, with saidbarge being located entirely between said port and starboard columns,and

lift said barge from the surface of the body of water, so that saidbarge is carried above the water solely by buoyant forces acting on saidsupport surface with said submersible hull members being floatinglydisposed in said body of water; and

prior to the termination of said method, rigidly weld ing a plurality ofextension frames between the upper portions of at least some of saidport and starboard columns and the port and starboard sides of saidbarge, respectively, with portions of said extension frames beingsubstantially flush with said working deck of said barge to define anextension of opposite sides of said working deck.

2. A method according to claim 1 wherein said step of providing saidbuoyancy means includes the steps of providing a plurality of ballastchambers and ballast pumping means in each hull, and communicating theballast pumping means of each hull by conduit means extending throughone of said brace beams.

3. A method according to claim 1 wherein the step of floating said bargeincludes the step of floating a barge having a rotary crane mounted on aworking deck thereof with the rotary crane axis extending generallythrough the centerline of said barge.

4. A method according to claim 1 and further including the step ofmounting a pivotal crane at least partially on the upper surface of oneof said extension frames.

5. A method according to claim 1 wherein the step of floating said bargeincludes the step of floating said barge such that the centerlinethereof is offset within said barge receiving zone; and said step ofrigidly welding includes the steps of welding a frame extension betweenthe barge and the support columns at one side of said cradle unit andarranging an upper horizontal surface of said frame extension to extendalong a substantial portion of the length of the barge generally flushwith a working deck portion of said barge.

6. A method of converting a floatable barge having a working deck into asemisubmersible lay vessel suitable for laying an elongate flexiblemeans into a body of water, said method comprising the steps of:

providing an independently floatable cradle unit having spacedhorizontally elongate submersible hull members defining port andstarboard submersible hull members,

a plurality of columns extending vertically up wardly from each of saidhull members to define port and starboard columns,

transverse bracing means interconnecting said port columns with saidstarboard columns, said bracing means including upper and lowertransverse bracing and intermediate diagonal bracing, and

an inclined lay ramp section supported by rampsupporting portions of thecolumns of one of said submersible hull members. the ramp-supportingportions being arranged at levels of rearwardly diminishing height inaccordance with the inclination of said ramp, and

buoyancy means for selectively ballasting and deballasting saidsubmersible hull members;

actuating said buoyancy means to cause said cradle unit to partiallysink within a body of water while maintaining said cradle unitpositively buoyant. such that said submersible hull members and saidtransverse bracing are completely submerged.

said upper transverse bracing defines a horizontally disposed supportsurface means disposed below the surface ofthe body of water between theport and starboard submersible hull members, and

the upper ends of at least some of the columns of said submersible hullmembers extend above the water surface to define a barge receiving zonetherebetween;

floating said barge on the body of water into said barge receiving zonebetween said zone-defining port and starboard columns to orient theunderside of said barge over said support surface means,

while maintaining said cradle unit positively buoyant;

actuating said buoyancy means for de-ballasting said hull members andincreasing the positive buoyancy thereof to:

raise said cradle unit in a manner bringing said support surface meansinto supporting contact with the underside of said barge, with saidbarge being located entirely between said port and starboard columns,and

lift said barge from the surface of the body of water, so that saidbarge is carried above the water solely by buoyant forces acting on saidsupport surface means with said submersible hull members beingfloatingly disposed in said body of water; and at least prior to thetermination of said method:

rigidiy welding upper portions of said rampsupporting columns to oneside of said barge so as to establish a lay ramp that extends from anupper location substantially flush with said working deck to a lowerposition disposed below the underside of said barge for guiding elongateflexible means from said barge, and rigidly welding a plurality ofextension frames between the upper portions of at least some of thecolumns located opposite said ramp supporting columns and the other sideof said barge, with portions of said extension frames beingsubstantially flush with said working deck of said barge to define anextension of said working deck. 7. A method according to claim 6including the step of attaching a stinger mechanism to said cradle unitat the aft-most column supporting said ramp.

1. A method of converting a floatable barge having a working deck into asemi-submersible vessel comprising the steps of: providing anindependently floatable cradle unit having spaced horizontally elongatesubmersible hull members defining port and starboard submersible hullmembers, a plurality of columns extending vertically upwardly from eachof said hull members to define port and starboard columns, transversebracing means interconnecting said port columns with said starboardcolumns, said bracing means comprising lower transverse bracing, uppertransverse bracing, and intermediate diagonal bracing, and buoyancymeans for selectively ballasting and de-ballasting said submersible hullmembers; actuating said buoyancy means to cause said cradle unit topartially sink within a body of water while maintaining said cradle unitpositively buoyant, such that said submersible hull members arecompletely submerged, said upper transverse bracing defines ahorizontally disposed support surface means disposed below the surfaceof the body of water between the port and starboard submersible hullmembers, and the upper ends of at least some of the columns of saidsubmersible hull members extend above the water surface to define abarge receiving zone therebetween; floating said barge on the body ofwater into said barge receiving zone between said port and starboardcolumns to orient the underside of said barge over said support surfacemeans, while maintaining said cradle unit positively buoyant; actuatingsaid buoyancy means for de-ballasting said hull members and increasingthe postive buoyancy thereof to: raise said cradle unit in a mannerbringing said support surface means into supporting contact with theunderside of said barge, with said barge being located entirely betweensaid port and starboard columns, and lift said barge from the surface ofthe body of water, so that said barge is carried above the water solelyby buoyant forces acting on said support surface with said submersiblehull members being floatingly disposed in said body of water; and priorto the termination of said method, rigidly welding a plurality ofextension frames between the upper portions of at least some of saidport and starboard columns and the port and starboard sides of saidbarge, respectively, with portions of said extension frames beingsubstantially flush with said working deck of said barge to define anextension of opposite sides of said working deck.
 2. A method accordingto claim 1 wherein said step of providing said buoyancy means includesthe steps of providing a plurality of ballast chambers and ballastpumping means in each hull, and communicating the ballast pumping meansof each hull by conduit means extending through one of said brace beams.3. A method according to claim 1 wherein the step of floating said bargeincludes the step of floating a barge having a rotary crane mounted on aworking deck thereof with the rotary crane axis extending generallythrough the centerline of said barge.
 4. A method according to claim 1and further including the step of mounting a pivotal crane at leastpartially on the upper surface of one of said extension frames.
 5. Amethod according to claim 1 wherein the step of floating said bargeincludes the step of floating said barge such that the centerlinethereof is offset within said barge receiving zone; and said step ofrigidly welding includes the steps of welding a frame extension betweenthe barge and the support columns at one side of said cradle unit andarranging an upper horizontal surface of said frame extension to extendalong a substantial portion of the length of the barge generally flushwith a working deck portion of said barge.
 6. A method of converting afloatable barGe having a working deck into a semi-submersible lay vesselsuitable for laying an elongate flexible means into a body of water,said method comprising the steps of: providing an independentlyfloatable cradle unit having spaced horizontally elongate submersiblehull members defining port and starboard submersible hull members, aplurality of columns extending vertically upwardly from each of saidhull members to define port and starboard columns, transverse bracingmeans interconnecting said port columns with said starboard columns,said bracing means including upper and lower transverse bracing andintermediate diagonal bracing, and an inclined lay ramp sectionsupported by ramp-supporting portions of the columns of one of saidsubmersible hull members, the ramp-supporting portions being arranged atlevels of rearwardly diminishing height in accordance with theinclination of said ramp, and buoyancy means for selectively ballastingand de-ballasting said submersible hull members; actuating said buoyancymeans to cause said cradle unit to partially sink within a body of waterwhile maintaining said cradle unit positively buoyant, such that saidsubmersible hull members and said transverse bracing are completelysubmerged, said upper transverse bracing defines a horizontally disposedsupport surface means disposed below the surface of the body of waterbetween the port and starboard submersible hull members, and the upperends of at least some of the columns of said submersible hull membersextend above the water surface to define a barge receiving zonetherebetween; floating said barge on the body of water into said bargereceiving zone between said zone-defining port and starboard columns toorient the underside of said barge over said support surface means,while maintaining said cradle unit positively buoyant; actuating saidbuoyancy means for de-ballasting said hull members and increasing thepositive buoyancy thereof to: raise said cradle unit in a mannerbringing said support surface means into supporting contact with theunderside of said barge, with said barge being located entirely betweensaid port and starboard columns, and lift said barge from the surface ofthe body of water, so that said barge is carried above the water solelyby buoyant forces acting on said support surface means with saidsubmersible hull members being floatingly disposed in said body ofwater; and at least prior to the termination of said method: rigidlywelding upper portions of said ramp-supporting columns to one side ofsaid barge so as to establish a lay ramp that extends from an upperlocation substantially flush with said working deck to a lower positiondisposed below the underside of said barge for guiding elongate flexiblemeans from said barge, and rigidly welding a plurality of extensionframes between the upper portions of at least some of the columnslocated opposite said ramp supporting columns and the other side of saidbarge, with portions of said extension frames being substantially flushwith said working deck of said barge to define an extension of saidworking deck.
 7. A method according to claim 6 including the step ofattaching a stinger mechanism to said cradle unit at the aft-most columnsupporting said ramp.